The painstaking job of cleaning 300 years of London's pollution and history from one of the most famous facades in the world is finally over.

On Thursday, an almost eerily white St Paul's cathedral was unveiled: it was the first day Sir Christopher Wren's masterpiece had spent without scaffolding for 15 years. The 18-month, final stage of the £40m restoration had led to the cathedral's west front being entirely hidden behind hoardings. But now the cathedral's steps and ceremonial entrance are on show once again.

Restoration work included transforming the blackened and damaged west front of the cathedral with more than 150,000 blocks of the cathedral's white Portland stone cleaned on the outside alone, to allow the patina of the building's history to remain inside.

Other work included cleaning and repair of the interior, redesign and landscaping of the south churchyard gardens, restoration of the grand organ and adding wheelchair access to the crypt. The American Memorial Chapel, built in the 1950s to commemorate US forces who died in the second world war, has also been cleaned and restored.

Cathedral officials said the interior had been "transformed" by state-of-the-art conservation techniques with mosaics, carvings, and sculpture brought to life by light "flooding" the building.

Martin Stancliffe, surveyor to the fabric, who oversaw the project, said: "This great building is now in a sound state and probably looks better than at any time since its completion in 1711."

A service will be held to celebrate the 300th anniversary on Tuesday. To mark the completion of the project, St Paul's is also running a competition where photographers are invited to submit their best exterior shots of the building.